showcase: extend rank history to 32 weeks, trim leading empty columns

Previously the graph was fixed at 5 weekly data points.  Now:

- rankHistoryPoints raised from 5 → 32 so the store accumulates up to 32
  weekly snapshots and each project carries 32 history entries.
- GenerateRankHistorySVG trims all leading all-zero columns before layout,
  so the graph only spans from the oldest week that any project actually
  has data — no empty space on the left when history is short.
- displayPoints (trimmed width) drives all layout math: xPos(), xLabels,
  and the X-axis loop; numPoints is only used for the reversal placement.
- xLabelStep() thins X-axis text labels when many columns are visible
  (step 1 for ≤6, 2 for ≤12, 4 for ≤24, 8 for >24 columns) while still
  drawing a grid line at every column and always labelling 'now'.
- Test: give alpha a Spot at '4w' so trimming keeps that column and the
  time-axis label test still finds '4w ago' in the output.
- New TestXLabelStep_ReturnsSensibleSteps covers the new helper.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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